The present invention relates to an apparatus and method for dimming a gas discharge lamp. More particularly, the invention relates to an apparatus and method for dimming a gas discharge lamp in which one or both of the dead time periods between drive pulses are varied or modulated.
Gas discharge lamps are highly efficient sources of light energy. Gas discharge lamps are typically formed of gas filled vessels having electrodes. In order to power the lamp, a high voltage is applied across the electrodes, usually at a high frequency. Gases within the tube become ionized as they are excited by high energy radiation. The light output of the lamp is thus directly related to the input power signal characteristics.
Dimming of gas discharge lamps is well known. A typical method of dimming a gas discharge lamp is disclosed in U.S. Pat. No. 5,045,758 issued Sep. 3, 1991 to Hildebrand which discloses a pulse width modulated alternating current drive signal for energizing the lamp and for controlling the light emission intensity of the lamp. In addition to such modulating of the pulse width or drive pulse of the lamp's drive signal, other prior art examples modulate the frequency of the drive signal. For example, U.S. Pat. No. 4,916,362 issued Apr. 10, 1990 to Orenstein discloses an excitation supply for gas discharge tubes using a variable frequency oscillator. By varying the frequency of the oscillator, the brightness of the gas discharge lamp may be selected. Finally, the prior art discloses signal modulation using various, often more complex and expensive, means. For example, U.S. Pat. No. 5,434,479 issued Jul. 18, 1995 to Matsushita discloses first and second series circuits each series circuit including two switching elements and a connecting point located therebetween wherein ON/OFF timing of the switching elements in the first series circuit varies between in-phase and 180 degrees out-of-phase from ON/OFF timing of the switching elements in the second series circuit. The ON period ratio of the switching elements is different in at least one of the first and second series circuits. The device thereby allows dimming of the discharge lamp. However, the Matsushita device requires a full bridge drive circuit composed of four switching elements.